The invention relates to a signal evaluation method for a fiber-optic rotation sensor utilizing the Sagnac effect and employing two superposed light waves rotating in opposite directions in a circuit formed of a plurality of windings of a light conductive fiber and wherein a phase modulator modulates the rotating light with a frequency f and an evaluation unit determines the rotational velocity from the signal formed by the rotation sensor.
Known optical rotation rate sensors utilize, without exception, an interferometric measuring principle. In that case, the influence of rotation on two electromagnetic waves (light waves) rotating oppositely to one another on a closed transmission path is detected and measured with the aid of the characteristic interference structure.
To attain high detection sensitivity, a phase sensitive detection technique is generally utilized which requires additional phase modulation. A drawback of this method is that the signal present at the end contains a proportionality factor in addition to the actual information (trigonometric function which is proportional to the rate of rotation). Each variation of this preliminary factor influences the stability of the scale factor of the rotation sensor, for example as a result of the following effects:
variation of the power emitted by the electromagnetic transmitter (e.g. aging of the laser diode) and thus also variation of the partial powers resulting from power division;
variation of transmission attenuation, particularly a change in the alignment state between the individual optical components, aging of the light conductive fiber, etc.;
so-called polarization fading due to statistical polarization rotation during passage through the propagation medium and the subsequent polarizers;
all effects which influence interference capability of the light waves and thus visibility of the interference structure, particularly variation in the length of coherence;
variation of the dividing ratio at the primary beam divider; a constant dividing ratio can be realized only if certain parameters are maintained (e.g. polarization direction);
variation of detector efficiency; for optical light receivers, this efficiency is subject to a typical aging behavior and is additionally highly dependent on environmental conditions (temperature, voltage supply).
A method is therefore disclosed in German Offenlegungsschrift DE-OS No. 3,040,514 in which the output signal is evaluated at several frequencies in such a way that the preliminary factor is eliminated with the aid of quotient formation. However, this method is very complicated since it requires the use of additional phase sensitive rectifiers.